As methods for evaluating the quality and performance of a fuel cell, the following methods (1) through (4) are conventionally known.
(1) Measurement of Tafel Plot (Tafel Gradient) while Varying Flow Rates of Fuel and Oxidizer
Though it is possible to evaluate the performance of a fuel cell as a whole with this evaluation method (1), it is not possible to specify a defective point and content with respect to each of components in the fuel cell.
(2) Measurement of Cole-Cole Plot
This method (2) is a transient analysis of electrode reaction. In an AC impedance method, a transfer function of the electrode reaction is determined in such a manner that a voltage (or current) signal is supplied to the electrode and then the resultant response current (or voltage) is compared to the sinusoidal input. Here, the Cole-Cole plot is corrected in this method (2) using a coefficient β as indicated by the following equation (I).Z=Rsol+Rct/{1+(j2πfRctCdl)β}  (1)
With this method (2), it is possible to obtain a certain degree of internal information of a fuel cell by supplying a measurement frequency.
However, the internal information is not directly related to the quality of the fuel cell. Further, the correlation between the internal information and the quality of the fuel cell involves many hypotheses. Accordingly, data of a particular measurement result is not uniquely interpreted. Therefore it is not possible to realize the measurement correctly.
Further, with the method (2), it is possible to specify a defective point, and a part (electrode, separator, electrolytic membrane, etc.) of the fuel cell that differs from another non-defective fuel cell, but it is not possible to specify in further detail where and to what extent the point and part is defective.
Further, with the method (2), it takes not less than 100 seconds to measure a fuel cell, and an internal state of the fuel cell, such as a gas supply rate and temperature, changes during the measurement. This causes a problem that either it is difficult to realize the measurement correctly or it is impossible to realize the measurement.
(3) Impedance Measurement by an AC Four-Terminal Method using a Specific Frequency
This measurement method (3) provides a measurement result of only one physical property value, and provides only one index for evaluating an overall state of a fuel cell.
(4) Current Interruption Method
This evaluation method (4) is implemented by instantaneously interrupting a cell load current, and then measuring a transient characteristic of voltage rise that is caused by an induced current generated in the instantaneous interruption. This measurement is performed to obtain information inside the cell, and to obtain, in particular, information of an interface of an electrolytic membrane.
With this evaluation method (4), it is possible to specify a defective point, and a part (electrode, separator, electrolytic membrane, etc.) of the fuel cell that differs from another non-defective fuel cell. In this evaluation method (4), however, a counter current (reverse bias) generated by interrupting the load damages the fuel cell.
In order to reduce the damage, also devised is a method (5) of rapidly lowering the current to a certain degree of value instead of completely interrupting the current.
However, a difference between the method (5) and the method (4) is only an extent of current to be interrupted. Namely, the method (5) is substantially the same with the current interruption method (4).
Tokukaisho 64-24366 (published on Jan. 26, 1989) discloses a method for detecting a defective cell in a phosphoric acid fuel cell.
This detection method is used for detecting whether or not a single cell or a block of cells as a unit is defective in a cell stack, and not for correctly evaluating the properties of a fuel cell.
As described above, the foregoing conventional methods have a problem that it is difficult to correctly evaluate the properties of a fuel cell.